The instant invention relates to double clad optical fiber optimized for use in, for example, fiber lasers and amplifiers, as well as methods of manufacture and uses therefor.
Optical amplifiers, and in particular the optically-pumped erbium doped fiber amplifier (EDFA), are widely used in fiberoptic transmission systems (see, for example, E. Desurvire, Erbium Doped Fiber Amplifiers, Wiley, New York, 1994). In a typical device, a weak 1550 nanometer (nm) optical signal and a strong 980 nm pump signal, both propagating in single-mode optical fiber, are combined by means of a fused dichroic coupler into one single-mode fiber. This fiber is then coupled to a single-mode erbium-doped fiber where the erbium ions absorb the pump radiation and provide gain at the signal wavelength. The result is that the output of the EDFA is an amplified replica of the input signal. Such amplifiers are useful for overcoming the various losses that occur in any fiberoptic transmission system.
In a conventional fiber amplifier, the pump source consists of a laser diode operating in a single transverse mode coupled to single-mode optical fiber. The power density at the output facet of the pump laser limits the amount of optical power that can be obtained from such devices. To increase the diode output power, it is necessary to increase the emitting area of the diode. Unfortunately, when this is done, the transverse mode structure of the resulting broad area laser becomes multimode, and the laser output is no longer sufficiently coherent to be coupled into a single-mode fiber. Such a diode output can, however, be coupled into a multimode fiber to provide an essentially incoherent source for pumping the amplifier. Such multimode fibers are typically round, since this shape is easier to fabricate than any alternative shape.
In a variation of this design, ytterbium may be added to the fiber (as taught in, for example, U.S. Pat. No. 5,225,925, to Grubb et al. issued Jul. 6, 1993). In the optimized fiber disclosed in the ""925 patent, energy absorbed by the ytterbium ions is efficiently transferred to the erbium ions. This results in a fiber with a much stronger, broader absorption than can be obtained in a singly-doped erbium fiber. An amplifier made from such fiber (a ytterbium-erbium doped fiber amplifier, or YEDFA), can be pumped with longer wavelength sources, such as a diode-pumped neodymium laser (see Grubb et al., Electronics Letters, 1991); output powers in excess of 4 watts (W) have been reported (Grubb et al. paper, TuG4 OFC 1996). The wavelengths of neodymium lasers used for this purpose has varied from 1064 nm in Nd:YAG to 1047 nm in Nd:YLF. Over this range in a typical fiber, the Yb absorption varies from 2 to 7 dB/m. For comparison, in the same fiber at 950 nm the absorption was 420 dB/m, and, at 975 nm, it was 2500 dB/m.
Techniques also exist for pumping an amplifier directly with multimode diodes. U.S. Pat. No. 3,808,549, issued Apr. 30, 1974, to Mauer discloses a design in which a small, strongly absorbing, single-mode core is embedded in a large, multimode waveguide. With all modes excited, the optical power density in such a double clad waveguide is nearly uniform across the waveguide aperture. Under these conditions, the average absorption coefficient is approximately equal to the absorption coefficient of the core, normalized by the area ratio of the two waveguides. Radiation propagating in modes that overlap the doped region will be preferentially absorbed, and some form of mode mixing is often required to maintain the uniform power distribution required to ensure that all the power in the multimode waveguide will eventually be absorbed by the core.
Using a double clad design of this type, Minelly et al. (IEEE Photonics Technology Letters, 5(3), 301-303, 1993) demonstrated a YEDFA pumped with a broad area laser diode. Minelly et al. used bulk optics to couple the output of a laser diode array to the double-clad fiber. Geometries using fused or reflective couplers similar to those used for conventional single-mode amplifiers can also be used. With some modification, for example, the filter wavelength division multiplexer (FWDM) made by E-Tek Dynamics, Inc. of San Jose, Calif. could be used as a multimode coupler. This, combined with a double clad gain fiber, would permit a multimode-pumped amplifier to be built in the same geometry as the conventional EDFA described above.
The fiber shown by Mauer was round with a concentric core, as was the fiber used by Minelly et al. This is a very inefficient shape for a double clad device. As noted by Snitzer et al. (U.S. Pat. No. 3,729,690, Apr. 24, 1973 and U.S. Pat. No. 4,815,079, Mar. 21, 1989), in a double clad fiber with radial symmetry, many of the modes in the multimode waveguide do not interact with, and are not absorbed by, a concentric core. This phenomenon can also be described by geometrical optics, where it would be observed that the vast majority of the guided rays are skew rays that never pass through the core. This problem is a result of radial symmetry and can be eliminated by perturbations that break this symmetry. Snitzer et al. proposed the use of an off-center, circular waveguide as well as a rectangular guide with two different transverse dimensions. Additionally, Lewis et al., in U.S. Pat. No. 5,418,880, and Muendel, in U.S. Pat. No. 5,533,163, teach the use of various space filling polygons. Such shapes are limited to triangles, certain symmetric quadrilaterals, and regular hexagons.
The techniques used to make fibers with these shapes generally resulted in polymer-clad fibers that were not round or which had a single-mode core that was not concentric with the fiber. Polymer-clad fibers are less stable thermally and mechanically than silica fibers, and they can be easily damaged by the pump radiation. Non-concentric fibers are difficult to align and splice. The fact that the fibers are not round makes it difficult to combine these fibers with more standard fiber components and to exploit the existing infrastructure of tools such as fiber cleavers, splicers, and ferrules that are optimized for use with round optical fibers. For practical applications, it is important to utilize a shape that can be surrounded by a thickness of low-index silica outer cladding with a round outer diameter. Typical 100/125 multimode fiber has a 12.5 micron (gm) cladding thickness. The maximum outer radius of the waveguide is constrained by the desired outer diameter minus the cladding thickness.
Multimode pump sources and couplers are also optimized for round fiber. To efficiently couple a round multimode pump fiber to a non-circular gain fiber, it is important that the pump fiber diameter be less than or equal to the minimum inner diameter of the low-index silica outer cladding. Any radial perturbation in such fibers will be constrained to an annular region whose inner diameter is limited by the pump fiber diameter and whose outer diameter is limited by the fiber outer diameter and cladding thickness. The constraints on actual fibers are such that the radial dimension of the waveguide can only vary by xc2x110%, with values as small as xc2x15% being preferable in some cases. There is therefore a need for fibers that appear externally as round, concentric, all-silica fibers, but which nonetheless have been sufficiently perturbed to allow efficient double-clad absorption to occur.
Finally, it is important to recognize that double clad fibers are not truly single mode fibers. The same perturbations that allow efficient absorption also ensure that many guided modes at the signal wavelength will have appreciable overlap with the fiber core. This is often not a problem in a fiber laser, because the modes that oscillate will be those that most efficiently overlap the core region. However, in an amplifier, signal power that is coupled into the multimode waveguide could give rise to signal distortions when this signal is accidentally amplified and coupled back into the output signal. There is clearly a need for fibers that prevent these parasitic processes from occurring.
Accordingly, there exists a need for enhanced optical fibers in which guided rays propagating along the length of the fiber are passed through the fiber core. The enhanced fiber should retain the preferred round shape to remain compatible with other fiber components, as noted above. Likewise, the core of the fiber should preferably be substantially in the center of the fiber.